A rear end cross member reinforcement structure of automobile constructed to have respective members forming a rear end cross member join together to form a closed cross-sectional shape such that an external force applied from outside can be effectively diffused, and to have a structure at a joined portion between both tip end portions of the rear end cross member and a quarter panel changed to reinforce a structural strength relative to the rear end cross member, such that the rear end cross member can fully withstand an external force when the external force such as torsion or the like is applied thereto, the rear end cross member reinforcement structure of automobile having a rear body including a rear end cross member arranged crosswise of the body and a quarter panel joining both tip end portions of the rear end cross member, wherein each tip end of an upper rear end cross member, a lower rear end cross member and an outer rear end cross member which form the rear end cross member is joined together to form a closed cross-sectional shape and both tip end portions of the rear end cross member is joined by the quarter panel to form another closed cross-sectional shape.